The present invention relates generally to sealing a cable splice closure with a low surface energy adhesive and more particularly to injecting the adhesive into end seals used with such closures.
There are various methods for joining or splicing communication cable ends together. In so doing, there are many important considerations such as the use of compatible materials, how many cables are being spliced, is the spliced cable to be buried in soil, immersed in water or suspended in the air, what heat source is required to make the joint, i.e., flammable gasses, will the splice need to be reopened and remade without interruption of working circuits, will the joint have sufficient mechanical strength and is the cost feasible?
Communication cables are typically constructed of a conductor bundle, surrounded by a metal strength and interference sheathing and an outer protective coating, typically of a low surface energy material such as polyethylene. When such cables are spliced and rejoined, the strength and integrity of the rejoined cable is critical. An enclosure or a closure body is used to sealingly surround the splice. The closure body is also typically formed of a low surface energy material.
One persistent problem in the use of splice closures involves the need for a complete seal about the splice or closure body. Many prior art splice closures accomplish sealing by providing a complex array of nuts and bolts, clamps, gaskets and heat shrink tubing, as well as potting gels and resins, in various combinations. Besides the fact that these closure methods require significant assembly time, the closures still often suffer leaks or ruptures, particularly along their seals. This problem is even more acute at the end seal where the closure is sealed to the cable jacket, and where even the slightest defect can result in the migration of moisture along the jacket or the inner surface of the closure. A lack of a complete (hermetic) seal can also be particularly detrimental for pressurized closures. Occasionally, these closures must be re-entered and re-spliced. Typically, however, re-entry into a closure requires disturbing the end seal which is sealed to the cable and the closure. Therefore, re-sealing after re-entry becomes an acute problem.
Although these seals may be strengthened by the use of adhesives, the adhesive bonds formed are normally weak due to the low surface energy of the material of the closure, the end seals and cables, typically polyethylene. End seals can be used with a fusion bond and with hot melts as an alternative bonding material. Hot melt is placed between the resistance wires, and the wires are heated to form a bond between the cables and the end seal surfaces. Hot melt bonding can be used with different end seal materials such as foams, elastomers and thermoplastics, but the bond strength is weaker than a fusion bond seal.
Adhesive bonding or the achievement of adhesion of coatings to low surface energy polymeric materials has been a technological problem since the inception of the use of such materials in industry. There are many descriptions of the problems with the adhesive bonding of low energy surfaces. The difficulty with adhesive bonding of such materials stems, in part, from the fact that these materials are deemed to be "van der Waals" solids. That is, the primary force for cohesion that is available between polymer chains is that due to van der Waals or "dispersion" forces. Low surface energy materials derive their strength from molecular entanglements, cross-linking, crystallization or some combination of these. The surface energy of a polymer is a reflection of the forces which hold the chains together and is therefore low for these materials. Examples of low surface energy polymers are polytetrafluoroethylene, polyethylene, polypropylene, silicones, etc.
One criterion for adhesive bonding is that the adhesive must come into intimate contact with the substrate. That is, the adhesive must completely "wet" the substrate. Low surface energy polymers are very difficult to wet by polar liquids because the polar liquids have a surface energy that is higher than that of the substrate. Most high strength adhesives are polar materials and hence their surface energy is too high to wet the surface of most polymers. If the surface is incompletely wet by an adhesive, there is a greater chance for interfacial voids and hence a weaker bond.
Another criterion for adhesive bonding is that the surface must be free of weak boundary layers. Commercial plastics usually contain a substantial amount of additives such as stabilizers and flow control agents. Also, with free radically polymerized materials, there is also a substantial fraction of low molecular weight polymer in addition to the high molecular weight portion. In general, these low molecular weight fractions exude to the surface and form weak boundary layers. These layers must be removed before the plastic can be effectively bonded or coated.
There is a substantial science and technology developed around the surface preparation of low surface energy plastics for adhesive bonding or coating. The methods which have been developed include flame treatment, corona discharge treatment, plasma treatment, oxidation by ozone, oxidation by oxidizing acids, sputter etching as well as coating with higher surface energy materials. This last method is also known as "priming" and may have to be preceded by one of the physical methods (e.g., corona discharge treatment) in order to have the primer adhere well to the surface.
In general, the surface preparation methods described above act to increase the surface energy of the polymer and/or eliminate weak boundary layers and may also increase surface roughness. The surface energy of these plastics is usually increased by the introduction of oxidized species into the surface. The elimination of weak boundary layers may take place by crosslinking and/or ablation of the exuded species. There is usually a trade-off between the oxidation process and the weak boundary layers removal process since over-oxidized materials may themselves form a weak boundary layer.
Very few of the methods described in the literature are useful for a wide range of plastics. In general, the treatment method or the priming means is usually quite specific for the type of plastic used. This is a severe limitation for the general user of adhesive bonding since many of the physical methods of surface treatment require substantial capital investment. Thus, there is a need for a simple, easy to use adhesive bonding method that is capable of adhering, without priming, to a wide range of plastics including those classed as "low surface energy" plastics.
An efficient, effective means for adhesively bonding low surface energy plastic substrates such as polyethylene and polypropylene has long been sought for the assembly and repair cable splice closures. Typically, such assembly and repair is performed in the field. Consequently, there has been a considerable and long felt need for a simple, easy to use adhesive that can readily and effectively bond mating cable splice closure surfaces together as well as bonding communication cables to the end seals and bonding the end seals to the closure.
While an adhesive that can bond low surface energy plastics is advantageous, the commercial utility of such an adhesive would be enhanced if the components of the adhesive were combined in a convenient mix ratio and could be easily carried to a job site and readily applied using conventional adhesive dispensers without the need for laborious pre-mixing of the various components of the adhesive. Thus, there is not only a need for an adhesive that can bond low surface energy plastics, but a need for such an adhesive that is pre-blended and can be easily carried and readily applied without a material reduction in storage stability or performance.
Unfortunately, a suitable solution to the problems associated with ease of installation, seal integrity and strength, has not been satisfactorily addressed by the prior art. Therefore, what is needed is an apparatus and a method for sealing cable splice closures with a low surface energy adhesive. It is also highly desirable to inject an adhesive for bonding the closure to the end seals and for bonding the end seals to the cable, and additionally providing apparatus to accomplish this so that re-entry into the closure does not disturb the seal integrity of the cable, end seal and closure.